Antenna phase center compensation for orbital assistance data

ABSTRACT

A method, apparatus, and system are disclosed for providing modified orbital assistance data to a mobile station to determine its location using global navigation satellite system (GNSS). The modified orbital assistance data may include predicted orbital information for the GNSS satellites combined with antenna phase center offset data for one or more GNSS satellites. The antenna phase center offset data may indicate an offset distance from the center of mass of the GNSS satellite to a position on an antenna of the respective GNSS satellite. The modified orbital assistance data may be in an earth-centered earth-fixed (ECEF) frame of reference and the antenna phase center offset data may be in a body-centered frame of reference.

BACKGROUND OF THE DISCLOSURE 1. Field of the Disclosure

Aspects of the disclosure relate generally to global navigationsatellite systems and the like.

2. Description of the Related Art

Global navigation satellite systems (GNSS) are used to determine aglobal position and/or location of any number of mobile stations. A GNSSmay include a constellation of orbiting satellites that each transmit atime-synchronized signal. A mobile station may receive thetime-synchronized signal from a number of GNSS satellites. Bydetermining a time of transmission associated with each receivedtime-synchronized signal and having knowledge of the location of each ofthe satellites that transmitted each received time-synchronized signal,the mobile station may determine its global location. The typicalresolution of GNSS systems is typically in the range of two to threemeters, however, a more accurate location determination is desired.

SUMMARY

A mobile station functioning in a GNSS may receive modified orbitalassistance data from a location assistance server to aid in determiningthe location and/or position of the mobile station. The modified orbitalassistance data may include predicted orbital information for the GNSSsatellites combined with antenna phase center offset data for each GNSSsatellite. The antenna phase center offset data may indicate an offsetdistance from the center of mass of the GNSS satellite to a position ofan apparent source of signal radiation from the respective GNSSsatellite. The modified orbital assistance data may be in anearth-centered earth-fixed (ECEF) frame of reference and the antennaphase center offset data may be in a body-centered frame of reference.

In one implementation, a method for providing modified orbitalassistance data to a mobile station to determine a location of themobile station includes selecting a first frequency of a globalnavigation satellite system (GNSS) positioning signal, obtaining orbitalassistance data for one or more GNSS satellites, where the orbitalassistance data is with respect to the center of mass of the one or moreGNSS satellites, and obtaining frequency-specific antenna phase centeroffset data for the one or more GNSS satellites for the first frequency.The method may further include determining the modified orbitalassistance data based on the orbital assistance data, and thefrequency-specific antenna phase center offset data and providing themodified orbital assistance data to the mobile station.

In another implementation, a server configured to provide modifiedorbital assistance data to a mobile station may comprise acommunications interface configured to communicate with the mobilestation, at least one memory, at least one processor coupled to thecommunications interface and the at least one memory, where the at leastone processor is configured to: select a first frequency of a globalnavigation satellite system (GNSS) positioning signal, obtain orbitalassistance data for one or more GNSS satellites, where the orbitalassistance data is with respect to the center of mass of the one or moreGNSS satellites, obtain frequency-specific antenna phase center offsetdata for the one or more GNSS satellites, determine the modified orbitalassistance data based on the orbital assistance data, and thefrequency-specific antenna phase center offset data, and provide themodified orbital assistance data to the mobile station.

In another implementation, a non-transitory computer-readable storagemedium may include instructions that, when executed by one or moreprocessors of a server, cause the server to perform operations includingselecting a first frequency of a global navigation satellite system(GNSS), obtaining orbital assistance data for one or more GNSSsatellites, wherein the orbital assistance data is with respect to thecenter of mass of the one or more GNSS satellites, obtainingfrequency-specific antenna phase center offset data for the one or moreGNSS satellites, determining the modified orbital assistance data basedon the orbital assistance data, and the frequency-specific antenna phasecenter offset data, and providing the modified orbital assistance datato a mobile station.

In another implementation, a method for determining a location of amobile station using orbital assistance data includes receivingsatellite positioning signals from a plurality of global navigationsatellite system (GNSS) satellites, receiving, from a server, a modifiedorbital assistance data, including orbital information modified todescribe an antenna phase center offset of each GNSS satellite withrespect to a first frequency, and determining the location of the mobilestation based on the modified orbital assistance data and the firstfrequency of the satellite positioning signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofvarious aspects of the disclosure and are provided solely forillustration of the aspects and not limitation thereof.

FIG. 1 is a block diagram of a communication system, according to someimplementations.

FIG. 2 shows an exemplary environment which may include the mobilestation, the location server, and GNSS satellites.

FIG. 3 depicts an exemplary simplified orbital map of a GNSS satellite.

FIG. 4 shows a flowchart for an example operation for providing modifiedorbital assistance data to a mobile station.

FIG. 5 shows a flowchart for example operation for using modifiedorbital assistance data to determine the location of a mobile station.

FIG. 6 shows a schematic block diagram illustrating certain exemplaryfeatures of a location assistance server, that is configured to providemodified orbital assistance data to a mobile station, as describedherein.

FIG. 7 shows a block diagram illustrating certain exemplary features ofa mobile station that is configured to perform positioning within awireless network.

DETAILED DESCRIPTION

Aspects of the disclosure are provided in the following description andrelated drawings directed to various examples provided for illustrationpurposes. Alternate aspects may be devised without departing from thescope of the disclosure. Additionally, well-known elements of thedisclosure will not be described in detail or will be omitted so as notto obscure the relevant details of the disclosure.

The words “exemplary” and/or “example” are used herein to mean “servingas an example, instance, or illustration.” Any aspect described hereinas “exemplary” and/or “example” is not necessarily to be construed aspreferred or advantageous over other aspects. Likewise, the term“aspects of the disclosure” does not require that all aspects of thedisclosure include the discussed feature, advantage, or mode ofoperation.

Those of skill in the art will appreciate that the information andsignals described below may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the description below may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof, depending inpart on the particular application, in part on the desired design, inpart on the corresponding technology, etc.

Further, many aspects are described in terms of sequences of actions tobe performed by, for example, elements of a computing device. It will berecognized that various actions described herein can be performed byspecific circuits (e.g., application specific integrated circuits(ASICs)), by program instructions being executed by one or moreprocessors, or by a combination of both. Additionally, the sequence(s)of actions described herein can be considered to be embodied entirelywithin any form of non-transitory computer-readable storage mediumhaving stored therein a corresponding set of computer instructions that,upon execution, would cause or instruct an associated processor of adevice to perform the functionality described herein. Thus, the variousaspects of the disclosure may be embodied in a number of differentforms, all of which have been contemplated to be within the scope of theclaimed subject matter. In addition, for each of the aspects describedherein, the corresponding form of any such aspects may be describedherein as, for example, “logic configured to” perform the describedaction.

Various implementations relate generally to modifying orbital assistancedata for a mobile station. The orbital assistance data may be modifiedby a location assistance server that is separate from the mobilestation. The mobile station may receive satellite positioning signalsfrom a plurality of global navigational satellites and use the modifiedorbital assistance data to more quickly may determine its location. Themodified orbital assistance information may include antenna phase centeroffset information to increase the accuracy of the determined location.

Particular implementations of the subject matter described in thisdisclosure can be implemented to realize one or more of the followingpotential advantages. In some implementations, by generating themodified mobile assistance data at the location assistance server, themobile station is able to determine a more accurate position withreduced computational effort.

FIG. 1 is a block diagram of a communication system 100, according tosome implementations. The communication system 100 may include anorbital assistance data provider 110, an antenna phase offset dataprovider 150, a location assistance server 130 and a mobile station 120.Although only one mobile station 120 is shown, in other implementationsthe communication system 100 may include any technically feasible numberof mobile stations. The mobile station 120 also may be referred to as astation, a user equipment, a subscriber station, a mobile unit, asubscriber unit, a wireless unit, a remote unit, a mobile device, awireless communication device, a wireless device, a remote device, amobile subscriber station, an access terminal, a mobile terminal, awireless terminal, a remote terminal, a handset, a user agent, a mobileclient, a client, or some other suitable terminology.

The mobile station 120 may determine its location by receiving satellitepositioning signals from a plurality of global navigation satellitesystem (GNSS) satellites (not shown for simplicity). Example GNSSinclude the Global Positioning System (GPS) operated by the UnitedStates Government, the Galileo system operated by the European Union,the BeiDou Navigation System operated by the Government of China, theGLONASS System operated by the Russian Federation, the Indian RegionalNavigation Satellite System (IRNSS) operated by the Government of India,the Quasi-Zenith Satellite System (QZSS) operated by the Government ofJapan, and the like. Satellites within a GNSS are referred to as aconstellation. Each satellite transmits (e.g., broadcasts) a satellitepositioning signal. The satellite positioning signals may besynchronized to a common time reference. The mobile station 120 receivesthe satellite positioning signals from a number of GNSS satellites thatare in-view of the mobile station 120. The mobile station 120 determinesthe time of flight of the satellite positioning signals and, based onthe known location of the GNSS satellites, determines its location.

A GNSS may transmit the satellite positioning signals through aplurality of frequencies or frequency bands. For example, GPS satellitepositioning signals are transmitted through a first frequency of 1575.42MHz (sometimes referred to as the L1 frequency), a second frequency of1227.60 MHz (sometimes referred to as the L2 frequency), and a thirdfrequency of 1176.45 MHz (sometimes referred to as the L5 frequency).Galileo satellite positioning signals are transmitted through 1575.42,1278.75, 1191.795, 1176.450, and 1207.14 MHz frequencies. Other GNSSsystems may use other frequencies.

The location assistance server 130 may provide a modified orbitalassistance data 131 to the mobile station 120. The modified orbitalassistance data 131 may include data regarding the predicted locationsof the GNSS satellites within the constellation as well as dataregarding a frequency-specific antenna phase offset associated with eachGNSS satellite.

The location assistance server 130 may receive orbital assistance data111 and/or predicted orbital parameter files that includes orbitalassistance data 111 from the orbital assistance data provider 110. Insome implementations, location assistance server 130 receives theorbital assistance data 111 via a network 162. The network 162 mayinclude, but is not limited to, a network that supports InternetProtocol (IP) connections (e.g., the Internet). The location assistanceserver 130 may optionally include an interface, e.g., secure filetransfer program (SFTP), for securely transferring the predicted orbitdata from orbital assistance data provider 110.

The orbital assistance data 111 may include predicted orbitalinformation for one or more satellites operating within a GNSSconstellation. In some aspects, the orbital assistance data provider 110may generate the orbital assistance data 111 periodically (e.g. everyfew hours) that is valid for an extended duration in time (e.g., 6 hoursor more). The orbital assistance data 111 may also include 3-Duncertainty values for predicted satellite coordinates, uncertainty ofpredicted satellite clock corrections, as well as an indication ofpredicted outages. The predicted orbital information may predict orbitsof the GNSS satellites with respect to the center of mass of thesatellites in an earth-centered earth-fixed (ECEF) frame of reference.

The location assistance server 130 may also receive antenna phase centeroffset data 151 from the antenna phase offset data provider 150. In someimplementations, location assistance server 130 receives the antennaphase center offset data 151 via a network 164 (which may be similar tothe network 162). The antenna phase center offset data 151 describes apoint or location on an antenna of a GNSS satellite (with respect to thecenter of mass of the GNSS satellite) that is an apparent source oftransmitted satellite positioning signal (e.g., an apparent source ofradiation with respect to the frequency of the satellite positioningsignal). Furthermore, a GNSS satellite may transmit satellitepositioning signals through more than one frequency. Thus, the antennaphase center offset data 151 may be frequency-specific and includeinformation for the different frequencies (for example, differentlocations on the antenna) supported by the GNSS satellite. The mobilestation 120 may determine a more accurate distance from the satellite byconsidering the origination point (e.g., transmission point) of thesatellite positioning signal on the antenna instead of the center ofmass of the associated GNSS satellite. In some implementations, usingthe antenna phase center offset data may improve the accuracy of thedetermined location to within five centimeters or less. The antennaphase center offset data 151 may be in a body-centered frame ofreference. The antenna phase offset data provider 150 may be anypublicly available source of antenna phase offset data such as, but notlimited to, the International GNSS service.

Conventionally, the mobile station 120 receives the orbital assistancedata 111 in the ECEF frame of reference and the antenna phase centeroffset data 151 in a body-centered frame of reference. In order tomodify the orbital assistance data 111 with the antenna phase centeroffset data 151, the mobile station translates the frame of reference ofboth the antenna phase center offset data 151 and the orbital assistancedata 111 to an earth-centered inertial (ECI) frame of reference. Themobile station 120 then combines the orbital assistance data 111 and theantenna phase center offset data 151 and convert the results back toECEF frame of reference. Alternatively, mobile station 120 can translateantenna phase center offset data 151 in body-centered frame to an ECEFframe of reference. This correction can then be applied to the orbitalassistance data 111 to obtain modified orbital assistance data 131.

The location assistance server 130 can off load the computational tasksdescribed above associated using the orbital assistance data 111 and theantenna phase center offset data 151 from the mobile station 120 byperforming the computations remotely (e.g., on the server) and providingthe modified orbital assistance data 131 that combines the orbitalassistance data 111 with the antenna phase center offset data 151. Forexample, the location assistance server 130 may translate the orbitalassistance data 111 from the ECEF frame of reference to an ECI frame ofreference, translate the antenna phase center offset data 151 from abody-centered frame of reference to the ECI frame of reference,determine a new (e.g., modified) orbital assistance data that modifiesthe orbital assistance data 111 with the antenna phase center offsetdata 151, and translate the modified orbital assistance data to the ECEFframe of reference. In some implementations, the modified orbitalassistance data may be referenced to the antenna phase center offsetdata 151 (e.g., the modified orbital assistance data may refer to theapparent source of radiation of the satellite positioning signalsinstead of the center of mass of the GNSS satellites). Furthermore,since a GNSS satellite may transmit satellite positioning signals inmore than one frequency, the location assistance server 130 may selectone of the frequencies and use the antenna phase center offset data 151associated with that frequency. The location assistance server 130 canprovide the modified orbital assistance data 131 to the mobile station120 through a network 166. The network 166 may be similar to the network162 and/or the network 164. The modified orbital assistance data 131 mayinclude an indication of the frequency selected by the locationassistance server 130.

Since the antenna phase center offset data 151 may also includeinformation that describes separate points on the antenna that may beassociated with other frequencies of a satellite positioning signal, thelocation assistance server 130 can generate an antenna phase centeroffset difference data 132 that describes the distance between the pointon the antenna associated with a first frequency of the satellitepositioning signal and a point on the antenna associated with a secondfrequency of satellite positioning signal. In some implementations, theantenna phase center offset difference data 132 may be in a radial,along-track, and cross-track frame of reference. In some otherimplementations, the antenna phase center offset difference data 132 maybe included with the modified orbital assistance data 131.

Although depicted as a separate network entity, in some implementationsthe location assistance server 130 may be any remote server, edge server(e.g. any feasible device in proximity to the mobile station 120), orbase station (e.g., any feasible eNodeB, gNodeB, or the like) coupled tothe mobile station 120.

FIG. 2 shows an exemplary environment 200 which may include the mobilestation 120, the location server 130 and GNSS satellites 201-205. TheGNSS satellites 201-205 may each transmit a respective satellitepositioning signal 211-215. The mobile station 120 may determine itslocation by receiving modified orbital assistance data 131 from thelocation server 130 and satellite positioning signals 211-215. Forexample, the mobile station 120 may determine the time of flight of thesatellite positioning signals 211-215, determine the locations of theGNSS satellites 201-205 using the modified orbital assistance data 131,and determine the location of the mobile station 120 that conforms tothe time of flight of the satellite positioning signals 211-215 and thelocations of the GNSS satellites 201-205. Notably, the satellitepositioning signals 211-215 may be transmitted simultaneously inmultiple frequencies or frequency bands. In some cases, the additionalfrequencies or frequency bands may provide the mobile station 120additional satellite positioning signals to receive and determine arespective time of flight.

The GNSS satellites 201-205 may be the GNSS satellites that are“in-view” of the mobile station 120. In other words, the mobile station120 may receive the satellite positioning signals 211-215 from the GNSSsatellites 201-205. The GNSS constellation may include other GNSSsatellites that are not in-view. That is, the navigation signalstransmitted by other GNSS satellites may not be received by the mobilestation 120. Although only five GNSS satellites are depicted in FIG. 2 ,in other implementations, the environment 200 may include any feasiblenumber of GNSS satellites.

Each GNSS satellite 201-205 may transmit a respective navigation signal211-215 that is received by the mobile station 120. The modified orbitalassistance data 131 may provide the mobile station 120 informationregarding the locations of the GNSS satellites 201-205. However, eachGNSS satellite 201-205 may have a unique antenna phase center offset dueto manufacturing and operational limitations. The modified orbitalassistance information 131 may incorporate the antenna phase centeroffset data 151 with the orbital assistance data 111 for each GNSSsatellite 201-205. In some implementations, the modified orbitalassistance data 131 may include assistance data from all GNSS satellitesincluded within the GNSS constellation. In other words, the modifiedorbital assistance data 131 may include assistance data for the in-viewsatellites as well as assistance data for GNSS satellite that are notin-view. The antenna phase offset center data 151 is described in moredetail with respect to FIG. 3 .

FIG. 3 depicts an exemplary simplified orbital map 300 of a GNSSsatellite 301. The orbital map 300 shows a single GNSS satellite 301 forsimplicity. In other implementations, the orbital map 300 may includeany number of GNSS satellites. Furthermore, the orbital map 300 showsthe GNSS satellite 301 in an orbit 302 around the Earth 303. The orbitalassistance data 111 may be determined with respect to the position ofthe Earth 303, and therefore, in an ECEF frame of reference.

The GNSS satellite 301 may have an associated center of mass 304. Theorbital assistance data 111 may be determined (e.g., computed) withrespect to the center of mass 304. Each GNSS satellite also has anantenna 305 to transmit the satellite positioning signals. Typically,the position of the antenna 305 is not coincident with the center ofmass 304, which may cause errors in ranging calculations that determinethe distance between the GNSS satellite 301 and the mobile station 120(not shown for simplicity). Furthermore, the satellite positioningsignal may effectively be transmitted from different locations on theantenna 305 based on the frequency associated with the satellitepositioning signal.

As described with respect to FIG. 1 , the antenna phase center offsetdata 151 describes the difference between the center of mass 304 and anapparent source of the satellite positioning signal on the antenna 305.Furthermore, the antenna phase center offset data 151 may also describedifferent locations (e.g., apparent positions) on the antenna 305 thatare associated with different frequencies of satellite positioningsignals. As shown, the antenna offset data 151 is conventionallydescribed in a body-centered frame of reference. The body-centered frameof reference is a frame of reference that is referenced to locationsalong or within the GNSS satellite 301 and is a function of satelliteposition in its respective orbit and/or orientation with respect to thesun.

The antenna phase center offset data 151 may be a constant offset in abody-centered frame of reference. However, the antenna phase centeroffset data 151 may be a function of time and the position of the sunwith respect to the GNSS satellite 301 in an earth-centered frame ofreference. For example, orientation of the GNSS satellite 301 may changeas the GNSS satellite 301 orbits around the Earth 303 to maintain anorientation of the solar panels of the GNSS satellite 301 to the sun(shown as by vector e_(D)). In other words, the antenna phase centeroffset data 151 may be based on the position of the GNSS satellite 301in its orbit 302 and a time of day.

The location server 130 can generate the modified orbital assistancedata 131 based on the orbital assistance data 111 and the antenna phasecenter offset data 151. Since the antenna phase center offset data 151is dependent on the frequency of the satellite positioning signal, thelocation server 130 may determine or select a frequency for which themodified orbital assistance data 131 is determined. The selectedfrequency may be specified by a user or selected based on conventionalor typical frequencies used by the GNSS satellites to transmit thesatellite positioning signal. In some implementations, the locationserver 130 may also determine the position of the sun with respect tothe GNSS satellite 301 as a function of time thereby taking into accountthe orbit of the GNSS satellite 301 and its orientation to the sun. Theorbital assistance data 111 and the modified orbital assistance data 131may be in the ECEF frame of reference and the antenna phase centeroffset data 151 may be in a body-centered frame of reference. Themodified orbital assistance data 131 may be referenced to the antennaphase center offset data 151.

The antenna phase center offset data 151 may also include data to locatean antenna phase center offset for other frequencies (e.g., frequenciesother than the selected frequency) of satellite positioning signal. Thelocation server 130 may determine an antenna phase offset differencethat describes the distance between the antenna phase offset dataassociated with the selected frequency and a second frequency. Forexample, if the L1 frequency is selected by the location server 130 togenerate the modified orbital assistance data 131, the location server130 may generate an antenna phase center offset difference data 132 thatdescribes the change in location on the antenna 305 that an L2 frequencysatellite positioning signal may be transmitted from. The antenna phasecenter offset difference data 132 may be in a radial, along-track, andcross-track frame of reference. The radial, along-track, and cross-trackframe of reference may be a moving frame of reference where the radialvector is toward the Earth 303, the along-track vector is in thedirection of the orbit 302, and the cross-track vector is orthogonal tothe radial and along-track vectors. By providing the antenna phasecenter offset difference data 132 in the radial, along-track, andcross-track frame of reference, the location server 130 enables themobile station 120 to easily and quickly adjust any ranging informationthat has been performed with the modified orbital assistance data 131.Furthermore, if the along-track and cross-track components of theantenna phase center offset difference data 132 are small relative tothe radial component, then the mobile station 120 may determine thedistance from the GNSS satellite 301 and the mobile station 120 byconsidering just the radial component of the antenna phase center offsetdifference data 132 and ignoring the along-track and cross-trackcomponents. For example, a distance between the GNSS satellite 301 andthe mobile station 120 may be easily and simply be offset by theaddition or subtraction of the antenna phase center offset differencedata 132. In some implementations, the location assistance server 130may approximate the actual antenna phase center offset difference datawith a best-fit mathematical function. In some other implementations,the antenna phase center offset difference data 132 may be included withthe modified orbital assistance data 131.

FIG. 4 shows a flowchart for an example operation 400 for providingmodified orbital assistance data 131 to a mobile station 120. Theoperation 400 is described below with respect to the location assistanceserver 130 of FIGS. 1 and 2 for illustrative purposes only. In otherimplementations, the operation 400 may be performed by any othertechnically feasible server or device.

At block 404, the location assistance server 130 obtains the orbitalassistance data 111 for one or more of the GNSS satellites. The orbitalassistance data 111 may include predicted orbital information for theone or more GNSS satellites. The orbital assistance data 111 may beprovided by any technically feasible source and may be in an ECEF frameof reference.

At block 406, the location assistance server 130 obtainsfrequency-specific antenna phase center offset data 151 for the one ormore GNSS satellites for a first frequency of a plurality of frequenciesof a GNSS positioning signal. The antenna phase center offset data 151may describe a location or position on the antenna of the GNSS satelliteassociated with the transmission of the satellite positioning signal ofthe selected frequency. The location may be an apparent source of theGNSS positioning signal on the antenna, where different locations areassociated with different frequencies of the GNSS positioning signal. Insome implementations, the antenna phase center offset data 151 mayspecify a distance from the center of mass of a GNSS satellite to aposition on an antenna of a respective GNSS satellite associated with anapparent source of radiation of the first frequency of the GNSSpositioning signal.

At block 408, the location assistance server 130 determines the modifiedorbital assistance data based on the orbital assistance data (obtainedat block 404) and the antenna phase center offset data (obtained atblock 406). The modified orbital assistance data may be in an ECEF frameof reference and may include an indication of the frequency associatedwith the frequency-specific antenna phase center offset data. In someimplementations, the modified orbital assistance data 131 may includeantenna phase center offset difference data which may indicate thedifference between antenna phase centers associated with different GNSSsatellite frequencies. In some implementations, the modified orbitalassistance data 131 may include the antenna phase center offsetdifference data 132 which may be approximated with a best-fitmathematical function.

In some implementations, the antenna phase center offset difference maybe in a radial, along-track, and cross-track frame of reference. Theantenna phase center offset difference is based on a position of the oneor more GNSS satellites within their respective orbits and a time ofday.

At block 410, the location assistance server 130 provides the modifiedorbital assistance data 131 to the mobile station 120. The modifiedorbital assistance data may be provided via a network (such as thenetwork 166 of FIG. 1 ) or by any other feasible mechanism.

FIG. 5 shows a flowchart for example operation 500 for using modifiedorbital assistance data 131 to determine the location of a mobilestation 120. The operation 500 is described below with respect to themobile station 120 of FIG. 1 for illustrative purposes only. Theoperation 500 also may be performed by any other feasible mobile stationor device.

At block 502, the mobile station 120 receives satellite positioningsignals from a plurality of GNSS satellites. The GNSS satellites maytransmit satellite positioning signals through two or more frequenciesor frequency bands. The mobile station may receive the satellitepositioning signals through at least one of the frequencies or frequencybands.

At block 504, the mobile station 120 receives, from a server, orbitalassistance data including orbital information for one or more GNSSsatellites referenced to a respective antenna phase center for a firstfrequency of the satellite positioning signals. In some implementations,the mobile station 120 may receive the modified orbital assistance data131 which may include orbital assistance data describing the predictedorbit information for one or more of the GNSS satellites that isreferenced to an associated antenna phase center for the firstfrequency. In some implementations, the orbital assistance data may bein an ECEF frame of reference.

In some implementations, the orbital assistance data may include antennaphase center offset difference data 132 for one or more frequenciesother than the first frequency. The antenna phase center offsetdifference data may indicate the difference between an apparent sourceof the first frequency of the satellite positioning signal and anapparent source of other frequencies of the satellite positioningsignal. The mobile station may select a second frequency of thesatellite positioning signals and determine the location of the mobilestation using the second frequency of the satellite positioning signalsand the antenna phase center offset difference data 132. In someimplementations, the antenna phase center offset difference data 132 fora second frequency of the satellite positioning signals may be in aradial, along-track, and cross-track frame of reference.

At block 506, the mobile station 120 may determine its location based onthe orbital assistance data and a first frequency of the satellitepositioning signals. In some implementations, the mobile station 120 maydetermine its location by using a frequency other than the firstfrequency, using radial antenna phase center offset difference data, andignoring along-track and cross-track antenna phase center offsetdifference data. In some implementations, the mobile station 120 maydetermine its location by using a combination of frequencies broadcastby one or more GNSS satellites and using the modified orbital assistancedata 131. In still other implementations, the antenna phase centeroffset difference data 132 for each of the one or more frequencies iswith respect to the first frequency.

FIG. 6 shows a schematic block diagram illustrating certain exemplaryfeatures of a location assistance server 600, that is configured toprovide modified orbital assistance data to a mobile station, asdescribed herein. Location assistance server 600 may, for example,include one or more processors 602, memory 604, an external interface,which may include a communications interface 618 (e.g., wireline orwireless network interface to base stations and/or entities in the corenetwork), which may be operatively coupled with one or more connections606 (e.g., buses, lines, fibers, links, etc.) to non-transitory computerreadable medium 620 and memory 604. In certain example implementations,all or part of location server 600 may take the form of a chipset,and/or the like.

The one or more processors 602 may be implemented using a combination ofhardware, firmware, and software. For example, the one or moreprocessors 602 may be configured to perform the functions discussedherein by implementing one or more instructions or program code 608 on anon-transitory computer readable medium, such as medium 620 and/ormemory 604. In some embodiments, the one or more processors 602 mayrepresent one or more circuits configurable to perform at least aportion of a data signal computing procedure or process related to theoperation of location server 600.

The medium 620 and/or memory 604 may store instructions or program code608 that contain executable code or software instructions that whenexecuted by the one or more processors 602 cause the one or moreprocessors 602 to operate as a special purpose computer programmed toperform the techniques disclosed herein. As illustrated in locationserver 600, the medium 620 and/or memory 604 may include one or morecomponents or modules that may be implemented by the one or moreprocessors 602 to perform the methodologies described herein. While thecomponents or modules are illustrated as software in medium 620 that isexecutable by the one or more processors 602, it should be understoodthat the components or modules may be stored in memory 604 or may bededicated hardware either in the one or more processors 602 or off theprocessors.

A number of software modules and data tables may reside in the medium620 and/or memory 604 and be utilized by the one or more processors 602in order to manage both communications and the functionality describedherein. It should be appreciated that the organization of the contentsof the medium 620 and/or memory 604 as shown in location server 600 ismerely exemplary, and as such the functionality of the modules and/ordata structures may be combined, separated, and/or be structured indifferent ways depending upon the implementation of the location server600.

The medium 620 and/or memory 604 may include a frequency selectionmodule 622 that, when implemented by the one or more processors 602,configures the one or more processors 602 to select a frequency from aplurality of frequencies associated with the satellite positioningsignals. The satellite positioning signals may be transmitted by a GNSSsatellite in more than one frequency. For example, when operating withGPS satellite positioning signals in the L1 and L2 frequency bands,execution of the frequency selection module 622 may cause the locationassistance server 600 to select either the L1 or the L2 frequency band.

The medium 620 and/or memory 604 may include an orbital assistance datareception module 624 that, when implemented by the one or moreprocessors 602, configures the one or more processors 602 to receiveand/or obtain orbital assistance data for one or more GNSS satellites.The orbital assistance data may describe predicted orbits for the GNSSsatellites with respect to their center of mass. The orbital assistancedata may be received and/or obtained from any technically feasiblesource.

The medium 620 and/or memory 604 may include an antenna phase centeroffset data reception module 626 that when implemented by the one ormore processors 602 configures the one or more processors 602 to receiveand/or obtain antenna phase center information. For example, executionof the antenna phase center offset data reception module 626 may causethe location assistance server 600 to receive antenna center phaseoffset data that describes points on the antenna of the GNSS satellitesthat maybe associated with the transmission of different frequencies ofsatellite positioning signals.

The medium 620 and/or memory 604 may include a modified orbitalassistance data computation module 628 that when implemented by the oneor more processors 602 configures the one or more processors 602 todetermine a modified orbital assistance data, such as the modifiedorbital assistance data 131 of FIG. 1 . In some implementations,execution of the modified orbital assistance data computation module 628may cause the location assistance server 130 to determine the modifiedorbital assistance data 131 as described with respect to FIG. 4 .

The medium 620 and/or memory 604 may include an antenna phase centeroffset difference module 632 that when implemented by the one or moreprocessors 602 configure the one or more processors 602 to determine adifference between phase center offset locations (e.g., positions) on anantenna of a GNSS satellite that are associated with differentfrequencies of the satellite positioning signals.

The methodologies described herein may be implemented by various meansdepending upon the application. For example, these methodologies may beimplemented in hardware, firmware, software, or any combination thereof.For a hardware implementation, the one or more processors 602 may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof.

For a firmware and/or software implementation, the methodologies may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. Any machine readable mediumtangibly embodying instructions may be used in implementing themethodologies described herein. For example, software codes may bestored in a non-transitory computer readable medium 620 or memory 604that is connected to and executed by the one or more processors 602.Memory may be implemented within the one or more processors or externalto the one or more processors. As used herein the term “memory” refersto any type of long term, short term, volatile, nonvolatile, or othermemory and is not to be limited to any particular type of memory ornumber of memories, or type of media upon which memory is stored.

If implemented in firmware and/or software, the functions may be storedas one or more instructions or program code 608 on a non-transitorycomputer readable medium, such as medium 620 and/or memory 604. Examplesinclude computer readable media encoded with a data structure andcomputer readable media encoded with a computer program 608. Forexample, the non-transitory computer readable medium including programcode 608 stored thereon may include program code 608 to provide modifiedorbital assistance data in a manner consistent with disclosedembodiments. Non-transitory computer readable medium 620 includesphysical computer storage media. A storage medium may be any availablemedium that can be accessed by a computer. By way of example, and notlimitation, such non-transitory computer readable media can compriseRAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic diskstorage or other magnetic storage devices, or any other medium that canbe used to store desired program code 608 in the form of instructions ordata structures and that can be accessed by a computer; disk and disc,as used herein, includes compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk and blu-ray disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer readable media.

In addition to storage on computer readable medium 620, instructionsand/or data may be provided as signals on transmission media included ina communication apparatus. For example, a communication apparatus mayinclude a wireless transceiver 610 having signals indicative ofinstructions and data. The instructions and data are configured to causeone or more processors to implement the functions outlined in theclaims. That is, the communication apparatus includes transmission mediawith signals indicative of information to perform disclosed functions.

Memory 604 may represent any data storage mechanism. Memory 604 mayinclude, for example, a primary memory and/or a secondary memory.Primary memory may include, for example, a random access memory, readonly memory, etc. While illustrated in this example as being separatefrom one or more processors 602, it should be understood that all orpart of a primary memory may be provided within or otherwiseco-located/coupled with the one or more processors 602. Secondary memorymay include, for example, the same or similar type of memory as primarymemory and/or one or more data storage devices or systems, such as, forexample, a disk drive, an optical disc drive, a tape drive, a solidstate memory drive, etc.

In certain implementations, secondary memory may be operativelyreceptive of, or otherwise configurable to couple to a non-transitorycomputer readable medium 620. As such, in certain exampleimplementations, the methods and/or apparatuses presented herein maytake the form in whole or part of a computer readable medium 620 thatmay include computer implementable code 608 stored thereon, which ifexecuted by one or more processors 602 may be operatively enabled toperform all or portions of the example operations as described herein.Computer readable medium 620 may be a part of memory 604.

An entity in a communication system, such as the location assistanceserver 600, may be configured to provide modified orbital assistancedata within the communication system and may include a means forselecting a first frequency of a GNSS positioning signal may be, e.g.,the one or more processors 602 with dedicated hardware or implementingexecutable code or software instructions in the memory 604 and/or medium620 such as the frequency selection module 622. A means for obtainingorbital assistance data from a one or more entities in the communicationsystem may be, e.g., the communications interface 618 and one or moreprocessors 602 with dedicated hardware or implementing executable codeor software instructions in memory 604 and/or medium 620 such as theorbital assistance data reception module 624. A means for obtainingfrequency-specific antenna phase center offset data from one or moreentities in the communication system may be, e.g., the communicationsinterface 618 and one or more processors 602 with dedicated hardware orimplementing executable code or software instructions in memory 604and/or medium 620 such as the antenna phase center offset data receptionmodule 626. A means for determining the modified orbital assistance datamay be, e.g., one or more processors 602 with dedicated hardware orimplementing executable code or software instructions in memory 604and/or medium 620 such as the modified orbital assistance datacomputation module 628.

In one implementation, the entity may be the location assistance server600 and may further include a means for determining antenna phase centeroffset difference of the one or more GNSS satellites for one or morefrequencies that differ from the first frequency, wherein the modifiedorbital assistance data includes the antenna phase center offsetdifference of the one or more GNSS satellites may be, e.g., one or moreprocessors 602 with dedicated hardware or implementing executable codeor software instructions in memory 604 and/or medium 620 such as theantenna phase center offset difference module 632.

FIG. 7 shows a block diagram illustrating certain exemplary features ofa mobile station 700 that is configured to perform positioning within awireless network. The mobile station 700 may be an example of the mobilestation 120 of FIGS. 1 and 2 . The mobile station 700 may, for example,include one or more processors 702, memory 704, an external interfacesuch as a at least one wireless transceiver 710 (e.g., wireless networkinterface), which may be operatively coupled with one or moreconnections 706 (e.g., buses, lines, fibers, links, etc.) to anon-transitory computer readable medium 720 and memory 704. The mobilestation 700 may further include additional items, which are not shown,such as a user interface that may include e.g., a display, a keypad orother input device, such as virtual keypad on the display, through whicha user may interface with mobile station 700. In certain exampleimplementations, all or part of mobile station 700 may take the form ofa chipset, and/or the like. Wireless transceiver 710 may, for example,include a transmitter 712 enabled to transmit one or more signals overone or more types of wireless communication networks and a receiver 714to receive one or more signals transmitted over the one or more types ofwireless communication networks.

In some implementations, mobile station 700 may include a mobile stationantenna 701, which may be internal or external. Mobile station antenna701 may be used to transmit and/or receive signals processed by wirelesstransceiver 710. In some embodiments, mobile station antenna 701 may becoupled to wireless transceiver 710.

In some implementations, the mobile station 700 may include a GNSSreceiver 715. The GNSS receiver 715 may receive satellite positioningsignals through the GNSS antenna 703 and determine the location of themobile station 700. In some other implementations, the one or moreprocessors 702 may receive time of flight information from the GNSSreceiver 715 and determine the location of the mobile station 700. TheGNSS receiver 715 and/or one or more processors 702 may use modifiedorbital assistance data 131 to determine the location of the mobilestation 700. In some implementations, functionality of the GNSS receiver715 may be included with, or provided by, the wireless transceiver 710.Thus, the wireless transceiver 710 may receive satellite positioningsignals and determine the location of the mobile station 700 in a mannersimilar to the GNSS receiver 715. The one or more processors 702 may beimplemented using a combination of hardware, firmware, and software. Forexample, the one or more processors 702 may be configured to perform thefunctions discussed herein by implementing one or more instructions orprogram code 708 on a non-transitory computer readable medium, such asmedium 720 and/or memory 704. In some embodiments, the one or moreprocessors 702 may represent one or more circuits configurable toperform at least a portion of a data signal computing procedure orprocess related to the operation of mobile station 700.

The medium 720 and/or memory 704 may store instructions or program code708 that contain executable code or software instructions that whenexecuted by the one or more processors 702, cause the one or moreprocessors 702 to operate as a special purpose computer programmed toperform the techniques disclosed herein. As illustrated in mobilestation 700, the medium 720 and/or memory 704 may include one or morecomponents or modules that may be implemented by the one or moreprocessors 702 to perform the methodologies described herein. While thecomponents or modules are illustrated as software in medium 720 that isexecutable by the one or more processors 702, it should be understoodthat the components or modules may be stored in memory 704 or may bededicated hardware either in the one or more processors 702 or off theprocessors.

A number of software modules and data tables may reside in the medium720 and/or memory 704 and be utilized by the one or more processors 702in order to manage both communications and the functionality describedherein. It should be appreciated that the organization of the contentsof the medium 720 and/or memory 704 as shown in mobile station 700 ismerely exemplary, and as such the functionality of the modules and/ordata structures may be combined, separated, and/or be structured indifferent ways depending upon the implementation of the mobile station700.

The medium 720 and/or memory 704 may include a GNSS signal receptionmodule 722 that, when implemented by the one or more processors 702,configures the one or more processors 702 and/or the GNSS receiver 715(or the wireless transceiver 710 providing the functionality of the GNSSreceiver 715) to receive a first frequency of satellite positioningsignals. In some implementations, execution of the GNSS signal receptionmodule 722 causes the mobile station 700 to determine the time of flightassociated with the received satellite positioning signals.

The medium 720 and/or memory 704 may include a modified orbitalassistance reception module 724 that when implemented by the one or moreprocessors 702 configures the one or more processors 702 and/or thewireless transceiver 710 to receive the modified orbital assistance data131

The medium 720 and/or memory 704 may include a location determinationmodule 726 that, when implemented by the one or more processors 702,configures the one or more processors 702 to determine a location of themobile station 700 based at least on GNSS satellite positioning signalsreceived by the GNSS receiver 715 (or the wireless transceiver 710providing the functionality of the GNSS receiver 715). In someimplementations, execution of the location determination module 726 maycause the mobile station 700 to determine the location of the mobilestation 700 based on received GNSS satellite positioning signals and themodified orbital assistance data 131. In some other implementations,execution of the location determination module 726 may cause the mobilestation 700 to determine the location of the mobile station 700 based onreceived GNSS satellite positioning signals and antenna phase centeroffset data that may be include with the modified orbital assistancedata 131.

The medium 720 and/or memory 704 may include a secondary frequencyselection module 728 that, when implemented by the one or moreprocessors 702, configures he one or more processors 702 to select asecond frequency of a GNSS satellite positioning signal. In someimplementations, execution of the secondary frequency selection module728 may cause the mobile station 700 to select a second GNSS satellitepositioning signal frequency to determine the location of the mobilestation 700.

The medium 720 and/or memory 704 may include an antenna phase centercorrection module 730 that, when implemented by the one or moreprocessors 702, configures the one or more processors 702 to determine adifference between at least two antenna phase centers (or example,associated with two different frequencies of satellite positioningsignals. In some implementations, the antenna phase center correctionmodule 730 may be implemented via a neural network. For example, anartificial or simulated neural network may be used to model variousantenna phase centers associated with various frequencies for one orGNSS satellites.

The medium 720 and/or memory 704 may include an orbital assistancemodification module 732 that, when implemented by the one or moreprocessors 702, configures the one or more processors 702 to modify themodified orbital assistance data 131 based on the determined differencebetween at least two antenna phase centers.

The methodologies described herein may be implemented by various meansdepending upon the application. For example, these methodologies may beimplemented in hardware, firmware, software, or any combination thereof.For a hardware implementation, the one or more processors 702 may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof.

For a firmware and/or software implementation, the methodologies may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. Any machine readable mediumtangibly embodying instructions may be used in implementing themethodologies described herein. For example, software codes may bestored in a non-transitory computer readable medium 720 or memory 704that is connected to and executed by the one or more processors 702.Memory may be implemented within the one or more processors or externalto the one or more processors. As used herein the term “memory” refersto any type of long term, short term, volatile, nonvolatile, or othermemory and is not to be limited to any particular type of memory ornumber of memories, or type of media upon which memory is stored.

If implemented in firmware and/or software, the functions may be storedas one or more instructions or program code 708 on a non-transitorycomputer readable medium, such as medium 720 and/or memory 704. Examplesinclude computer readable media encoded with a data structure andcomputer readable media encoded with a computer program 708. Forexample, the non-transitory computer readable medium including programcode 708 stored thereon may include program code 708 to supportdetermining the location and/or position of a mobile station in a mannerconsistent with disclosed embodiments. Non-transitory computer readablemedium 720 includes physical computer storage media. A storage mediummay be any available medium that can be accessed by a computer. By wayof example, and not limitation, such non-transitory computer readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to store desired program code 708 in theform of instructions or data structures and that can be accessed by acomputer; disk and disc, as used herein, includes compact disc (CD),laser disc, optical disc, digital versatile disc (DVD), floppy disk andblu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveshould also be included within the scope of computer readable media.

In addition to storage on computer readable medium 720, instructionsand/or data may be provided as signals on transmission media included ina communication apparatus. For example, a communication apparatus mayinclude a wireless transceiver 710 having signals indicative ofinstructions and data. The instructions and data are configured to causeone or more processors to implement the functions outlined in theclaims. That is, the communication apparatus includes transmission mediawith signals indicative of information to perform disclosed functions.

Memory 704 may represent any data storage mechanism. Memory 704 mayinclude, for example, a primary memory and/or a secondary memory.Primary memory may include, for example, a random access memory, readonly memory, etc. While illustrated in this example as being separatefrom one or more processors 702, it should be understood that all orpart of a primary memory may be provided within or otherwiseco-located/coupled with the one or more processors 702. Secondary memorymay include, for example, the same or similar type of memory as primarymemory and/or one or more data storage devices or systems, such as, forexample, a disk drive, an optical disc drive, a tape drive, a solidstate memory drive, etc.

In certain implementations, secondary memory may be operativelyreceptive of, or otherwise configurable to couple to a non-transitorycomputer readable medium 720. As such, in certain exampleimplementations, the methods and/or apparatuses presented herein maytake the form in whole or part of a computer readable medium 720 thatmay include computer implementable code 708 stored thereon, which ifexecuted by one or more processors 702 may be operatively enabled toperform all or portions of the example operations as described herein.Computer readable medium 720 may be a part of memory 704.

An entity in a communication system, such as mobile station 700, may beconfigured to determine the location of the mobile station 700 and mayinclude a means for receiving GNSS satellite positioning signals, whichmay be, e.g., the GNSS receiver 715 (or the wireless transceiver 710providing the functionality of the GNSS receiver 715) and one or moreprocessors 702 with dedicated hardware or implementing executable codeor software instructions in memory 704 and/or medium 720 such as theGNSS signal reception module 722. A means for receiving modified orbitalassistance data may be, e.g., the wireless transceiver 710 and one ormore processors 702 with dedicated hardware or implementing executablecode or software instructions in memory 704 and/or medium 720 such asthe modified orbital assistance reception module 724. A means fordetermining the location of the mobile station 700 may be, e.g., thewireless transceiver 710 and one or more processors 702 with dedicatedhardware or implementing executable code or software instructions inmemory 704 and/or medium 720 such as the location determination module726.

In one implementation, the entity may be the mobile station 700 and mayfurther include a means for selecting a second frequency of a satellitepositioning signal may be, e.g., one or more processors 702 withdedicated hardware or implementing executable code or softwareinstructions in memory 704 and/or medium 720 such as the secondaryfrequency selection module 728. A means for determining the location ofthe mobile station 700 using the second frequency of the satellitepositioning signals and the antenna phase center offset difference datamay be, e.g., one or more processors 702 with dedicated hardware orimplementing executable code or software instructions in memory 704and/or medium 720 such as the location determination module 726. A meansfor determining an antenna phase center correction based on the antennaphase center offset difference data may be, e.g., one or more processors702 with dedicated hardware or implementing executable code or softwareinstructions in memory 704 and/or medium 720 such as the antenna phasecenter correction module 730. A means for modifying the orbitalassistance data based on the antenna phase center correction may be,e.g., one or more processors 702 with dedicated hardware or implementingexecutable code or software instructions in memory 704 and/or medium 720such as the orbital assistance modification module 732.

Reference throughout this specification to “one example”, “an example”,“certain examples”, or “exemplary implementation” means that aparticular feature, structure, or characteristic described in connectionwith the feature and/or example may be included in at least one featureand/or example of claimed subject matter. Thus, the appearances of thephrase “in one example”, “an example”, “in certain examples” or “incertain implementations” or other like phrases in various placesthroughout this specification are not necessarily all referring to thesame feature, example, and/or limitation. Furthermore, the particularfeatures, structures, or characteristics may be combined in one or moreexamples and/or features.

Some portions of the detailed description included herein are presentedin terms of algorithms or symbolic representations of operations onbinary digital signals stored within a memory of a specific apparatus orspecial purpose computing device or platform. In the context of thisparticular specification, the term specific apparatus or the likeincludes a general purpose computer once it is programmed to performparticular operations pursuant to instructions from program software.Algorithmic descriptions or symbolic representations are examples oftechniques used by those of ordinary skill in the signal processing orrelated arts to convey the substance of their work to others skilled inthe art. An algorithm is here, and generally, is considered to be aself-consistent sequence of operations or similar signal processingleading to a desired result. In this context, operations or processinginvolve physical manipulation of physical quantities. Typically,although not necessarily, such quantities may take the form ofelectrical or magnetic signals capable of being stored, transferred,combined, compared, or otherwise manipulated. It has proven convenientat times, principally for reasons of common usage, to refer to suchsignals as bits, data, values, elements, symbols, characters, terms,numbers, numerals, or the like. It should be understood, however, thatall of these or similar terms are to be associated with appropriatephysical quantities and are merely convenient labels. Unlessspecifically stated otherwise, as apparent from the discussion herein,it is appreciated that throughout this specification discussionsutilizing terms such as “processing,” “computing,” “calculating,”“determining” or the like refer to actions or processes of a specificapparatus, such as a special purpose computer, special purpose computingapparatus or a similar special purpose electronic computing device. Inthe context of this specification, therefore, a special purpose computeror a similar special purpose electronic computing device is capable ofmanipulating or transforming signals, typically represented as physicalelectronic or magnetic quantities within memories, registers, or otherinformation storage devices, transmission devices, or display devices ofthe special purpose computer or similar special purpose electroniccomputing device.

In the preceding detailed description, numerous specific details havebeen set forth to provide a thorough understanding of claimed subjectmatter. However, it will be understood by those skilled in the art thatclaimed subject matter may be practiced without these specific details.In other instances, methods and apparatuses that would be known by oneof ordinary skill have not been described in detail so as not to obscureclaimed subject matter.

The terms, “and”, “or”, and “and/or” as used herein may include avariety of meanings that also are expected to depend at least in partupon the context in which such terms are used. Typically, “or” if usedto associate a list, such as A, B or C, is intended to mean A, B, and C,here used in the inclusive sense, as well as A, B or C, here used in theexclusive sense. In addition, the term “one or more” as used herein maybe used to describe any feature, structure, or characteristic in thesingular or may be used to describe a plurality or some othercombination of features, structures, or characteristics. Though, itshould be noted that this is merely an illustrative example and claimedsubject matter is not limited to this example.

While there has been illustrated and described what are presentlyconsidered to be example features, it will be understood by thoseskilled in the art that various other modifications may be made, andequivalents may be substituted, without departing from claimed subjectmatter. Additionally, many modifications may be made to adapt aparticular situation to the teachings of claimed subject matter withoutdeparting from the central concept described herein.

In some implementations, a method (1) for providing modified orbitalassistance data to a mobile station to determine a location of themobile station, the method performed by a server and comprising:obtaining orbital assistance data for one or more global navigationsatellite system (GNSS) satellites, wherein the orbital assistance datais with respect to a center of mass of the one or more GNSS satellites,obtaining frequency-specific antenna phase center offset data for theone or more GNSS satellites for a first frequency for a plurality offrequencies of a GNSS positioning signal, determining the modifiedorbital assistance data based on the orbital assistance data, and thefrequency-specific antenna phase center offset data, and providing themodified orbital assistance data to the mobile station.

There may be some implementations (2) of the above method (1), whereinthe orbital assistance data is in an earth-centered earth-fixed (ECEF)frame of reference and the frequency-specific antenna phase centeroffset data is in a body-centered frame of reference.

There may be some implementations (3) of the above method (1), whereinthe frequency-specific antenna phase center offset data indicates anoffset distance from a center of mass of a GNSS satellite to a positionon an antenna of a respective GNSS satellite associated with an apparentsource of radiation of the first frequency of the GNSS positioningsignal.

There may be some implementations (4) of the above method (1), whereinthe modified orbital assistance data includes an indication of the firstfrequency of the GNSS positioning signal.

There may be some implementations (5) of the above method (1), furthercomprising determining an antenna phase center offset difference of theone or more GNSS satellites for one or more frequencies that differ fromthe first frequency, wherein the modified orbital assistance dataincludes the antenna phase center offset difference of the one or moreGNSS satellites.

There may be some implementations (6) of the above method (5), whereinthe antenna phase center offset difference of the one or more GNSSsatellites is in a radial, along-track, and cross-track frame ofreference.

There may be some implementations (7) of the above method (5), whereinthe antenna phase center offset difference is based on a position of theone or more GNSS satellites within their respective orbits and a time ofday.

There may be some implementations (8) of the above method (5), whereinthe antenna phase center offset difference is based on a best-fitmathematical approximation of actual antenna phase offset differences.

There may be some implementations (9) of the above method (1), whereinthe modified orbital assistance data includes an orbital position forone or more GNSS satellites in an ECEF frame of reference and referencedto the frequency-specific antenna phase center for the first frequencyselected of the GNSS positioning signal.

In some implementations, a server (10) configured to provide modifiedorbital assistance data to a mobile station, comprising: a memory, acommunications interface configured to communicate with the mobilestation, one or more processors operably coupled to the communicationsinterface and the memory, the one or more processors configured to:obtain orbital assistance data for one or more global navigationsatellite system (GNSS) satellites, wherein the orbital assistance datais with respect to a center of mass of the one or more GNSS satellites,obtain frequency-specific antenna phase center offset data for the oneor more GNSS satellites for a first frequency for a plurality offrequencies of a GNSS positioning signal, determine the modified orbitalassistance data based on the orbital assistance data, and thefrequency-specific antenna phase center offset data, and provide themodified orbital assistance data to the mobile station.

There may be some implementations (11) of the server (10), wherein theorbital assistance data is in an earth-centered earth-fixed (ECEF) frameof reference and the frequency-specific antenna phase center offset datais in a body-centered frame of reference.

There may be some implementations (12) of the server (10), wherein thefrequency-specific antenna phase center offset data indicates an offsetdistance from a center of mass of a GNSS satellite to a position on anantenna of a respective GNSS satellite associated with an apparentsource of radiation of the first frequency of the GNSS positioningsignal.

There may be some implementations (13) of the server (10), wherein themodified orbital assistance data includes an indication of the firstfrequency of the GNSS positioning signal.

There may be some implementations (14) of the server (10), wherein theone or more processors are further configured to determine an antennaphase center offset difference of the one or more GNSS satellites forone or more frequencies that differ from the first frequency, whereinthe modified orbital assistance data includes the antenna phase centeroffset difference of the one or more GNSS satellites.

There may be some implementations (15) of the server (14), wherein theantenna phase center offset difference of the one or more GNSSsatellites is in a radial, along-track, and cross-track frame ofreference.

There may be some implementations (16) of the server (14), wherein theantenna phase center offset difference is based on a position of the oneor more GNSS satellites within their respective orbits and a time ofday.

There may be some implementations (17) of the server (14), wherein theantenna phase center offset difference is based on a best-fitmathematical approximation of actual antenna phase offset differences.

There may be some implementations (18) of the server (10), wherein themodified orbital assistance data includes an orbital position for one ormore GNSS satellites in an ECEF frame of reference and referenced to afrequency-specific antenna phase center for the first frequency of theGNSS positioning signal.

In some implementations, non-transitory computer-readable storage medium(19) comprising instructions that, when executed by one or moreprocessors of a server, cause the server to perform operationscomprising: obtaining orbital assistance data for one or more globalnavigation satellite system (GNSS) satellites, wherein the orbitalassistance data is with respect to a center of mass of the one or moreGNSS satellites, obtaining frequency-specific antenna phase centeroffset data for the one or more GNSS satellites for a first frequencyfor a plurality of frequencies of a GNSS positioning signal, determininga modified orbital assistance data based on the orbital assistance data,and the frequency-specific antenna phase center offset data, andproviding the modified orbital assistance data to a mobile station.

There may be some implementations (20) of the non-transitorycomputer-readable storage medium (19), wherein the orbital assistancedata is in an earth-centered earth-fixed (ECEF) frame of reference andthe frequency-specific antenna phase center offset data is in abody-centered frame of reference.

There may be some implementations (21) of the non-transitorycomputer-readable storage medium (19), wherein the frequency-specificantenna phase center offset data indicates an offset distance from acenter of mass of a GNSS satellite to a position on an antenna of arespective GNSS satellite associated with an apparent source ofradiation of the first frequency of the GNSS positioning signal.

There may be some implementations (22) of the non-transitorycomputer-readable storage medium (19), wherein the modified orbitalassistance data includes an indication of the first frequency of theGNSS positioning signal.

There may be some implementations (23) of the non-transitorycomputer-readable storage medium (19) further comprising: determining anantenna phase center offset difference of the one or more GNSSsatellites for one or more frequencies that differ from the firstfrequency, wherein the modified orbital assistance data includes theantenna phase center offset difference of the one or more GNSSsatellites.

There may be some implementations (24) of the non-transitorycomputer-readable storage medium (23), wherein the antenna phase centeroffset difference of the one or more GNSS satellites is in a radial,along-track, and cross-track frame of reference.

There may be some implementations (25) of the non-transitorycomputer-readable storage medium (23), wherein the antenna phase centeroffset difference is based on a position of the one or more GNSSsatellites within their respective orbits and a time of day.

There may be some implementations (26) of the non-transitorycomputer-readable storage medium (23) wherein the antenna phase centeroffset difference is based on a best-fit mathematical approximation ofactual antenna phase offset differences.

There may be some implementations (27) of the non-transitorycomputer-readable storage medium (19), wherein the modified orbitalassistance data includes an orbital position for one or more GNSSsatellites in an ECEF frame of reference and referenced to afrequency-specific antenna phase center for the first frequency selectedof the GNSS positioning signal.

In some implementations, a server (28) configured to provide modifiedorbital assistance data to a mobile station, comprising: means forobtaining orbital assistance data for one or more global navigationsatellite system (GNSS) satellites, wherein the orbital assistance datais with respect to a center of mass of the one or more GNSS satellites,means for obtaining frequency-specific antenna phase center offset datafor the one or more GNSS satellites for a first frequency for aplurality of frequencies of a GNSS positioning signal, means fordetermining the modified orbital assistance data based on the orbitalassistance data, and the frequency-specific antenna phase center offsetdata, and means for providing the modified orbital assistance data tothe mobile station.

There may be some implementations (29) of the server (28), wherein theorbital assistance data is in an earth-centered earth-fixed (ECEF) frameof reference and the frequency-specific antenna phase center offset datais in a body-centered frame of reference.

There may be some implementations (30) of the server (28), wherein thefrequency-specific antenna phase center offset data indicates an offsetdistance from a center of mass of a GNSS satellite to a position on anantenna of a respective GNSS satellite associated with an apparentsource of radiation of the first frequency of the GNSS positioningsignal.

There may be some implementations (31) of the server (28), wherein themodified orbital assistance data includes an indication of the firstfrequency of the GNSS positioning signal.

There may be some implementations (32) of the server (28), furthercomprising means for determining an antenna phase center offsetdifference of the one or more GNSS satellites for one or morefrequencies that differ from the first frequency, wherein the modifiedorbital assistance data includes the antenna phase center offsetdifference of the one or more GNSS satellites.

There may be some implementations (33) of the server (32), wherein theantenna phase center offset difference of the one or more GNSSsatellites is in a radial, along-track, and cross-track frame ofreference.

There may be some implementations (34) of the server (32), wherein theantenna phase center offset difference is based on a position the one ormore GNSS satellites within their respective orbits and a time of day.

There may be some implementations (35) of the server (32), wherein theantenna phase center offset difference is based on a best-fitmathematical approximation of actual antenna phase offset differences.

There may be some implementations (36) of the server 28, the modifiedorbital assistance data includes an orbital position for the one or moreGNSS satellites in an ECEF frame of reference and referenced to afrequency-specific antenna phase center for the first frequency selectedof the GNSS positioning signal.

Implementation examples are described in the following numbered clauses:

1. A method for determining a location of a mobile station using orbitalassistance data, the method comprising: receiving satellite positioningsignals from a plurality of global navigation satellite system (GNSS)satellites; receiving, from a server, the orbital assistance data,including orbital information for one or more GNSS satellites referencedto a respective antenna phase center for a first frequency of thesatellite positioning signals; and determining the location of themobile station based on the orbital assistance data and the satellitepositioning signals.

2. The method of clause 1, wherein the orbital assistance data is in anearth-centered earth-fixed (ECEF) frame of reference.

3. The method of any of clauses 1-2, wherein the orbital assistance datacomprises an antenna phase center offset difference data for one or morefrequencies that differ from the first frequency for the one or moreGNSS satellites, wherein determining the location of the mobile stationfurther comprises: selecting a second frequency of the satellitepositioning signals; and determining the location of the mobile stationusing the second frequency of the satellite positioning signals and theantenna phase center offset difference data.

4. The method of clause 3, wherein the antenna phase center offsetdifference data for the second frequency is in a radial, along-track,and cross-track frame of reference.

5. The method of clause 4, wherein determining the location of themobile station comprises using radial antenna phase center offsetdifference data and ignoring along-track and cross-track antenna phasecenter offset difference data.

6. The method of any of clauses 3-5, wherein the antenna phase centeroffset difference data for each of the one or more frequencies is withrespect to the first frequency.

7. The method of any of clauses 3-6, wherein the second frequency isdifferent than the first frequency.

8. The method of any of clauses 3-7, wherein the antenna phase centeroffset difference data includes an offset distance between a firstapparent source of radiation on an antenna of a GNSS satelliteassociated with the first frequency and a second apparent source ofradiation on the antenna associated with the second frequency.

9. The method of any of clauses 3-8, wherein determining the location ofthe mobile station comprises: determining an antenna phase centercorrection based on the antenna phase center offset difference data; andmodifying the orbital assistance data based on the antenna phase centercorrection.

10. A mobile station configured to determine its location based onglobal navigation satellite system (GNSS) positioning signalscomprising: a memory; a wireless transceiver configured to communicatethrough one or more wireless networks; one or more processors operablycoupled to the wireless transceiver and the memory, the one or moreprocessors configured to one or more processors operably coupled to thewireless transceiver and the memory, the one or more processorsconfigured to: receive satellite positioning signals from a plurality ofGNSS satellites; receive, from a server, an orbital assistance data,including orbital information for one or more GNSS satellites referencedto a respective antenna phase center for a first frequency of thesatellite positioning signals; and determine the location of the mobilestation based on the orbital assistance data and the satellitepositioning signals.

11. The mobile station of clause 10, wherein the orbital assistance datais in an earth-centered earth-fixed (ECEF) frame of reference.

12. The mobile station of any of clauses 10-11, wherein the orbitalassistance data comprises an antenna phase center offset difference datafor one or more frequencies that differ from the first frequency for theone or more GNSS satellites, wherein the determination of the locationof the mobile station further comprises the one or more processorsconfigured to: select a second frequency of the satellite positioningsignals; and determine the location of the mobile station using thesecond frequency of the satellite positioning signals and the antennaphase center offset difference data.

13. The mobile station of clause 12, wherein the antenna phase centeroffset difference data for the second frequency is in a radial,along-track, and cross-track frame of reference.

14. The mobile station of clause 13, wherein the determination of thelocation of the mobile station comprises using radial antenna phasecenter offset difference data and ignoring along-track and cross-trackantenna phase center offset difference data.

15. The mobile station of any of clauses 12-14, wherein the antennaphase center offset difference data for each of the one or morefrequencies is with respect to the first frequency.

16. The mobile station of any of clauses 12-15, wherein the antennaphase center offset difference data includes an offset distance betweena first apparent source of radiation on an antenna of a GNSS satelliteassociated with the first frequency and a second apparent source ofradiation on the antenna associated with the second frequency.

17. The mobile station of any of clauses 12-16, wherein thedetermination of the location of the mobile station further comprisesthe one or more processors configured to: determine an antenna phasecenter correction based on the antenna phase center offset differencedata; and modify the orbital assistance data based on the antenna phasecenter correction.

18. A non-transitory computer-readable storage medium comprisinginstructions that, when executed by one or more processors of a mobilestation, cause the mobile station to perform operations comprising:receiving satellite positioning signals from a plurality of globalnavigation satellite system (GNSS) satellites; receiving, from a server,an orbital assistance data, including orbital information for one ormore GNSS satellites referenced to a respective antenna phase center fora first frequency of the satellite positioning signals; and determininga location of the mobile station based on the orbital assistance dataand the satellite positioning signals.

19. The non-transitory computer-readable storage medium of clause 18,wherein the orbital assistance data is in an earth-centered earth-fixed(ECEF) frame of reference.

20. The non-transitory computer-readable storage medium of any ofclauses 18-19, wherein the orbital assistance data comprises an antennaphase center offset difference data for one or more frequencies thatdiffer from the first frequency for the one or more GNSS satellites,wherein determining the location of the mobile station furthercomprises: selecting a second frequency of the satellite positioningsignals; and determining the location of the mobile station using thesecond frequency of the satellite positioning signals and the antennaphase center offset difference data.

21. The non-transitory computer-readable storage medium of clause 20,wherein the antenna phase center offset difference data for the secondfrequency is in a radial, along-track, and cross-track frame ofreference.

22. The non-transitory computer-readable storage medium of clause 21,wherein determining the location of the mobile station comprises usingradial antenna phase center offset difference data and ignoringalong-track and cross-track antenna phase center offset difference data.

23. The non-transitory computer-readable storage medium of any ofclauses 20-22, wherein the antenna phase center offset difference datafor each of the one or more frequencies is with respect to the firstfrequency.

24. The non-transitory computer-readable storage medium of any ofclauses 20-23, wherein the second frequency is different than the firstfrequency.

25. The non-transitory computer-readable storage medium of any ofclauses 20-24, wherein the antenna phase center offset difference dataincludes an offset distance between a first apparent source of radiationon an antenna of a GNSS satellite associated with the first frequencyand a second apparent source of radiation on the antenna associated withthe second frequency.

26. The non-transitory computer-readable storage medium of any ofclauses 20-25, wherein determining the location of the mobile stationcomprises: determining an antenna phase center correction based on theantenna phase center offset difference data; and modifying the orbitalassistance data based on the antenna phase center correction.

27. A mobile station configured to determine its location based onglobal navigation satellite system (GNSS) positioning signalscomprising: means for receiving satellite positioning signals from aplurality of global navigation satellite system (GNSS) satellites, meansfor receiving, from a server, an orbital assistance data, includingorbital information for one or more GNSS satellites referenced torespective antenna phase center with respect to a first frequency; andmeans for determining the location of the mobile station based on theorbital assistance data and the satellite positioning signals.

28. The mobile station of clause 27, wherein the orbital assistance datacomprises an antenna phase center offset difference data for one or morefrequencies that differ from the first frequency for the one or moreGNSS satellites, wherein determining the location of the mobile stationfurther comprises: selecting a second frequency of the satellitepositioning signals; and determining the location of the mobile stationusing the second frequency of the satellite positioning signals and theantenna phase center offset difference data.

29. The mobile station of any of clauses 27-28, wherein the antennaphase center offset difference data for the second frequency is in aradial, along-track, and cross-track frame of reference.

30. The mobile station of clause 29, wherein determining the location ofthe mobile station comprises using radial antenna phase center offsetdifference data and ignoring along-track and cross-track antenna phasecenter offset difference data.

Therefore, it is intended that claimed subject matter not be limited tothe particular examples disclosed, but that such claimed subject mattermay also include all aspects falling within the scope of appendedclaims, and equivalents thereof.

What is claimed is:
 1. A method for determining a location of a mobilestation using orbital assistance data, the method comprising: receivingsatellite positioning signals from a plurality of global navigationsatellite system (GNSS) satellites; receiving, from a server, theorbital assistance data, including orbital information for one or moreGNSS satellites referenced to a respective antenna phase center for afirst frequency of the satellite positioning signals, and antenna phasecenter offset difference data representative of a distance between alocation on an antenna which is associated with the first frequency anda location on the antenna which is associated with a second frequency ofthe satellite positioning signals; and determining the location of themobile station based on the orbital assistance data and the satellitepositioning signals.
 2. The method of claim 1, wherein the orbitalassistance data is in an earth-centered earth-fixed (ECEF) frame ofreference.
 3. The method of claim 1, wherein the antenna phase centeroffset difference data is for one or more frequencies that differ fromthe first frequency for the one or more GNSS satellites, whereindetermining the location of the mobile station further comprises:selecting a second frequency of the satellite positioning signals; anddetermining the location of the mobile station using the secondfrequency of the satellite positioning signals and the antenna phasecenter offset difference data.
 4. The method of claim 3, wherein theantenna phase center offset difference data is in a radial, along-track,and cross-track frame of reference.
 5. The method of claim 4, whereindetermining the location of the mobile station comprises using radialantenna phase center offset difference data and ignoring along-track andcross-track antenna phase center offset difference data.
 6. The methodof claim 3, wherein the antenna phase center offset difference data foreach of the one or more frequencies is with respect to the firstfrequency.
 7. The method of claim 3, wherein the second frequency isdifferent than the first frequency.
 8. The method of claim 3, whereinthe antenna phase center offset difference data includes an offsetdistance between a first apparent source of radiation on an antenna of aGNSS satellite associated with the first frequency and a second apparentsource of radiation on the antenna associated with the second frequency.9. The method of claim 3, further comprising: determining an antennaphase center correction based on the antenna phase center offsetdifference data.
 10. A mobile station configured to determine itslocation based on global navigation satellite system (GNSS) positioningsignals comprising: a memory; a wireless transceiver configured tocommunicate through one or more wireless networks; a receiver; one ormore processors operably coupled to the wireless transceiver and thememory, the one or more processors configured to: receive, via thereceiver, satellite positioning signals from a plurality of GNSSsatellites; receive, from a server via the wireless transceiver, anorbital assistance data, including orbital information for one or moreGNSS satellites referenced to a respective antenna phase center for afirst frequency of the satellite positioning signals, and antenna phasecenter offset difference data representative of a distance between alocation on an antenna which is associated with the first frequency anda location on the antenna which is associated with a second frequency ofthe satellite positioning signals; and determine the location of themobile station based on the orbital assistance data and the satellitepositioning signals.
 11. The mobile station of claim 10, wherein theorbital assistance data is in an earth-centered earth-fixed (ECEF) frameof reference.
 12. The mobile station of claim 10, wherein the antennaphase center offset difference data is for one or more frequencies thatdiffer from the first frequency for the one or more GNSS satellites,wherein the determination of the location of the mobile station furthercomprises the one or more processors configured to: select a secondfrequency of the satellite positioning signals; and determine thelocation of the mobile station using the second frequency of thesatellite positioning signals and the antenna phase center offsetdifference data.
 13. The mobile station of claim 12, wherein the antennaphase center offset difference data is in a radial, along-track, andcross-track frame of reference.
 14. The mobile station of claim 13,wherein the determination of the location of the mobile stationcomprises using radial antenna phase center offset difference data andignoring along-track and cross-track antenna phase center offsetdifference data.
 15. The mobile station of claim 12, wherein the antennaphase center offset difference data for each of the one or morefrequencies is with respect to the first frequency.
 16. The mobilestation of claim 12, wherein the antenna phase center offset differencedata includes an offset distance between a first apparent source ofradiation on an antenna of a GNSS satellite associated with the firstfrequency and a second apparent source of radiation on the antennaassociated with the second frequency.
 17. The mobile station of claim12, wherein the one or more processors configured to: determine anantenna phase center correction based on the antenna phase center offsetdifference data, wherein the determination of the location of the mobilestation is based on the antenna phase center correction.
 18. A mobilestation configured to determine its location based on global navigationsatellite system (GNSS) positioning signals comprising: means forreceiving satellite positioning signals from a plurality of globalnavigation satellite system (GNSS) satellites; means for receiving anorbital assistance data, including orbital information for one or moreGNSS satellites referenced to respective antenna phase center withrespect to a first frequency of the satellite positioning signals, andantenna phase center offset difference data representative of a distancebetween a location on an antenna which is associated with the firstfrequency and a location on the antenna which is associated with asecond frequency of the satellite positioning signals; and means fordetermining the location of the mobile station based on the orbitalassistance data and the satellite positioning signals.
 19. The mobilestation of claim 18, wherein the orbital assistance data is in anearth-centered earth-fixed (ECEF) frame of reference.
 20. The mobilestation of claim 19, wherein the antenna phase center offset differencedata is for one or more frequencies that differ from the first frequencyfor the one or more GNSS satellites, wherein determining the location ofthe mobile station further comprises: selecting a second frequency ofthe satellite positioning signals; and determining the location of themobile station using the second frequency of the satellite positioningsignals and the antenna phase center offset difference data.
 21. Themobile station of claim 20, wherein the antenna phase center offsetdifference data is in a radial, along-track, and cross-track frame ofreference.
 22. The mobile station of claim 21, wherein the means fordetermining the location of the mobile station comprises means for usingradial antenna phase center offset difference data and ignoringalong-track and cross-track antenna phase center offset difference data.23. The mobile station of claim 20, wherein the antenna phase centeroffset difference data for each of the one or more frequencies is withrespect to the first frequency.
 24. The mobile station of claim 20,wherein the second frequency is different than the first frequency. 25.The mobile station of claim 20, wherein the antenna phase center offsetdifference data includes an offset distance between a first apparentsource of radiation on an antenna of a GNSS satellite associated withthe first frequency and a second apparent source of radiation on theantenna associated with the second frequency.
 26. The mobile station ofclaim 20, wherein the means for determining the location of the mobilestation comprises: means for determining an antenna phase centercorrection based on the antenna phase center offset difference data,wherein the means for determining the location of the mobile station isbased on the antenna phase center correction.
 27. A non-transitorycomputer-readable storage medium comprising instructions are configuredto, when executed by one or more processors of a mobile station, causethe mobile station to perform operations comprising: receiving satellitepositioning signals from a plurality of global navigation satellitesystem (GNSS) satellites; receiving, from a server, an orbitalassistance data, including orbital information for one or more GNSSsatellites referenced to a respective antenna phase center for a firstfrequency of the satellite positioning signals, and antenna phase centeroffset difference data representative of a distance between a locationon an antenna which is associated with the first frequency and alocation on the antenna which is associated with a second frequency ofthe satellite positioning signals; and determining a location of themobile station based on the orbital assistance data and the satellitepositioning signals.
 28. The non-transitory computer-readable storagemedium of claim 27, wherein the antenna phase center offset differencedata is for one or more frequencies that differ from the first frequencyfor the one or more GNSS satellites, wherein determining the location ofthe mobile station further comprises: selecting a second frequency ofthe satellite positioning signals; and determining the location of themobile station using the second frequency of the satellite positioningsignals and the antenna phase center offset difference data.